Have a look at this section of the documentation It features a partial screenshot of the manager page. The first row is the base image (various sizes), the second and third row are the additional images. If you click the add button (the one to the right which is cut off, so you can't actually see it), you are able to specify another additional image. You can delete the images, too or upload a different image instead of it.
Cheers,
Tim
edit: You can provide custom suffixes or have them automatically created. No need to rename your files prior to uploading them as additional images.
edit: As long as you don't want images with different content to show for medium and large images, you needn't worry about _MED and _LRG extensions.
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